U.S. patent number 4,615,691 [Application Number 06/559,865] was granted by the patent office on 1986-10-07 for surgically-implantable stepping motor.
Invention is credited to Carlos A. Hakim, Salomon Hakim.
United States Patent |
4,615,691 |
Hakim , et al. |
October 7, 1986 |
Surgically-implantable stepping motor
Abstract
A stepping motor isolated physically from electrical power
sources and powered by the influence of a magnetic field applied
from outside the apparatus, the stepping motor including a rotor
and one or more stator elements, the stator elements being composed
of magnetically soft and permeable material shaped and positioned
with respect to the rotor so that when magnetized under the
influence of the external field the stator elements strengthen and
orient the magnetic field in their vicinities so as to cause
movement of the rotor.
Inventors: |
Hakim; Carlos A. (Fort
Lauderdale, FL), Hakim; Salomon (Bogota, CO) |
Family
ID: |
24235370 |
Appl.
No.: |
06/559,865 |
Filed: |
December 8, 1983 |
Current U.S.
Class: |
604/9; 310/181;
310/40MM |
Current CPC
Class: |
A61F
2/0036 (20130101); H02K 37/00 (20130101); A61M
27/006 (20130101); A61M 27/002 (20130101) |
Current International
Class: |
A61F
2/00 (20060101); A61M 27/00 (20060101); H02K
37/00 (20060101); A61M 005/00 () |
Field of
Search: |
;604/8-10
;128/1.3-1.5,DIG.25 ;3/1.7 ;310/103,104,181,4MM
;335/138,139,272,306 ;137/539,539.5 ;261/DIG.74 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Conrad, Coleman W., "Stepping Motors", Machine Design, Apr. 11,
1974, pp. 78-80..
|
Primary Examiner: Rosenbaum; C. Fred
Assistant Examiner: Costantino; Mario
Attorney, Agent or Firm: Lee; G. Roger
Claims
We claim:
1. Surgically-implantable apparatus such as a valve, pump, or
pacemaker, said apparatus including a stepping motor for adjusting
said apparatus, said stepping motor being isolated physically from
electrical power sources and powered by the influence of an
external magnetic field applied from outside the apparatus, said
stepping motor comprising
a rotor,
one or more stator elements,
means for mounting said rotor so that it is capable of movement
relative to said stator elements,
said stator elements being composed of magnetically soft and
permeable material shaped and positioned with respect to said rotor
so that when magnetized under the influence of said external field
the said stator elements strengthen and orient the magnetic field
in their vicinities so as to cause incremental movement of said
rotor,
means for positioning said rotor with respect to said stator
elements so that when said stator elements are magnetized under the
influence of said external field incremental movement of said rotor
occurs,
wherein said apparatus is adapted to be surgically-implanted and
said magnetic field is applied from outside the body.
2. The apparatus of claim 1 wherein said rotor has pemanently
magnetic regions.
3. The apparatus of claim 1 or 2 wherein there are a plurality of
said stator elements spaced angularly around the perimeter of said
rotor.
4. The apparatus of claim 1 or 3 wherein there is one stator
element having a plurality of lobes extending from a central
region, said lobes being spaced angularly around the perimeter of
said rotor.
5. The apparatus of claim 1 combined with external apparatus for
generating said externally applied magnetic filed, said external
apparatus comprising a plurality of electromagnets and control
means for selectively energizing said electromagnets in a sequence
that causes said stator elements to be magnetized in successive
angular steps causing the rotor to move.
6. The apparatus of claim 5 wherein said electromagnets have the
same angular spacing around a central axis as said stator elements
have around the rotor axis so that said external apparatus can be
positioned with said axes coincident and with said electromagnets
at the same angular orientations as said stator elements.
7. The apparatus of claim 1 wherein said stator elements include
electrical coils, said coils being positioned with respect to said
stator elements so that an electrical current induced in said coils
by said magnetic field magnetizes said stator elements.
8. The apparatus of claim 1 or 7 wherein there are either a
plurality of said stator elements or one stator element with
separated lobes and wherein said stator elements or lobes are
positioned so that particular regions thereof have greatest effect
on movement of said rotor, and wherein said stator elements are
positioned so that by variation in orientation in said externally
applied magnetic field selected ones of said regions can be made
either all one polarity or all the other polarity or split between
both polarities, whereby the regions of one polarity can dominate
over the regions of split polarity in controlling movements of said
rotor.
9. The apparatus of claim 8 wherein there are a plurality of said
stator elements and each is positioned so that the radially inner
half thereof is closest to the magnetically active regions of said
rotor and thereby forms said region of greatest influence.
10. The apparatus of claim 9 wherein there are at least four said
stator elements equally spaced around the perimeter of said rotor
so that under the influence of an external magnetic field two
radially opposing elements have said regions of all one polarity
and the other two elements have said regions of split polarity.
11. The apparatus of claim 1 wherein said apparatus is an implanted
cerebrospinal fluid shunt valve and comprises a mechanism for
adjusting the pressure of said valve and means for connecting said
rotor to said mechanism so that movement of said rotor in response
to said applied external magnetic field causes adjustment of the
pressure of said valve.
12. The apparatus of claim 11 wherein said valve includes a valve
element and said adjustment mechanism comprises a cam adapted to
rotate with said rotor and a spring with two portions, one portion
engaging said valve element to provide a preload thereon and
another portion engaging said cam, and means for mounting said
spring so that said preload varies in response to rotation of said
cam.
13. The apparatus of claim 1 wherein said stator elements are
magnetically anisotropic so that they are more strongly magnetized
along the radial direction with respect to said rotor.
14. The method of surgically implanting a device and later
adjusting its operation from outside the body, comprising
surgically implanting a device having a stepping motor, said
stepping motor being physically isolated from electrical power
sources and comprising a rotor and one or more stator elements,
said stator elements being composed of magnetically soft and
permeable material shaped and positioned with respect to said rotor
so that when magnetized the local magnetic fields in their
vicinities cause rotation of said rotor,
applying from outside the body a magnetic field so as to magnetize
said stator elements to cause movement of said rotor.
15. A stepping motor for producing incremental movement within a
structure, said motor comprising
an external unit for placement outside said structure, said
external unit comprising one or more members for generating a
time-varying external magnetic field,
an internal unit for placement inside said structure, said internal
unit comprising
a rotor,
one or more stator elements each composed of a magnetically soft
and permeable material,
said stator elements being spaced from said rotor by a gap,
means for mounting said rotor so that it is capable of movement
relative to said stator elements,
said stoator elements being shaped and positioned with respect to
said rotor so that when magnetized under the influence of said
external magnetic field said stator elements strengthen and orient
the magnetic field in their vicinities so as to cause incremental
rotation of said rotor,
means for positioning said rotor with respect to said stator
elements so that when said stator elements are magnetized under the
influence of said external field incremental movement of said rotor
occurs,
said external and internal units being so constructed that said
external unit may be positioned remotely from said internal unit
during use, and
said external and internal units being so constructed that in use
the spacing of said field-generating members in said external unit
from said stator elements is greater than said gap between said
stator elements and said rotor.
16. The stepping motor of claim 15 wherein said internal unit is
adapted to be surgically-implanted and said external unit is
adapted to be used from outside the body.
17. The stepping motor of claim 15 wherein said members are
electromagnetic that remain fixed in place and means are provided
for selectively energizing different ones of said electromagnets to
generate said time-varying magnetic field.
18. The apparatus of claim 2 or 15 wherein means are provided for
mounting said rotor for rotation.
Description
BACKGROUND OF THE INVENTION
The invention relates to stepping motors, particularly those for
use in surgically-implanted devices.
Stepping motors have been known for many years. The simplest
stepping motor consists of a permanent magnet rotor surrounded by a
stator made up of four electromagnets. By selectively energizing
the electromagnets, it is possible to turn the rotor in angular
"steps" of 90.degree.. Often the rotor has a plurality of permanent
magnets, so as to reduce the angular step size. Some stepping
motors replace the permanent magnets on the rotor with regions of
different magnetic reluctance; in these, known as variable
reluctance motors, the rotor turns to the position of minimum
reluctance. Still others, known as hybrids, combine reluctance
differences with permanent magnets.
Stepping motors have been used in some medical applications. For
example, they have been used in medical infusion pumps for
delivering precise volumetric dosages of drugs at prescribed time
intervals. In all these applications the stepping motor was located
outside the patient's body, either in a portable device carried by
the patient or in a bedside unit.
Other types of medical devices (e.g., pacemakers) have been
implanted in the body. These have typically relied for a power
source on batteries implanted along with the device.
SUMMARY OF THE INVENTION
In general our invention features a new stepping motor that can be
operated while totally isolated physically from any source of
electrical power (i.e., without batteries and without wire
connections to an external power source). The usual electromagnets
used for the stator are replaced by pieces of magnetically soft and
permeable material (e.g., pure iron or special alloys such as
vacoperm). An externally applied magnetic field is used to
magnetize the stator elements so that the localized magnetic field
in the vicinity of the stator elements causes rotation of the
rotor. The external magnetic field need not be applied with great
precision, as the stator elements strengthen and orient the effect
of the magnetic field in their vicinities.
In preferred embodiments, the stepping motor of the invention is
installed in a surgically-implanted device and the magnetic field
is applied from outside the body; the rotor has a plurality of
permanently magnetic poles; there are a plurality of stator
elements spaced around the rotor or one stator element with a
plurality of lobes so spaced; the external magnetic field is
applied using apparatus having a plurality of electromagnets, which
are equal in number to the stator elements (or lobes of one
element) and positioned similarly; and the stator elements are
composed either of soft and permeable material or of such material
wrapped by an electrical coil (in which is induced an electrical
current, which in turn helps magnetize the soft, permeable stator
material).
The invention has the advantage of allowing a mechanical movement
(e.g., rotary or linear) to be induced within a device (e.g., to
alter the pressure setting of a valve, to activate a switch, or to
vary the parameter of an electrical circuit) without any physical
connection (e.g., electrical wires) with the device.
In the medical field, it makes possible implanting a device with an
internal element whose position can be accurately adjusted without
the need for any wires, tubes, or other physical elements
penetrating through the skin, or the use of implanted batteries.
Possible applications include cerebrospinal fluid shunt valves
(wherein the working pressure may be noninvasively adjusted very
precisely) and implantable pumps for precise volumetric delivery of
drugs.
The invention has the further advantage of being resistant to any
influence from the strong magnetic fields generated by nuclear
magnetic resonance (NMR) devices. Exposure to the fields of such
devices will generally cause only a small incremental movement of
the implanted stepping motor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Drawings
FIG. 1 is a perspective, somewhat diagrammatic, view of a first
preferred embodiment of the invention.
FIG. 2 is a cross-sectional view taken at 2--2 of FIG. 1, showing
the internal construction of said first embodiment.
FIG. 3 is a cross-sectional view taken at 3--3 in FIG. 2.
FIG. 4 is an elevation view taken at 4--4 in FIG. 2.
FIG. 5 is a plan view at 5--5 in FIG. 2, showing the cam of said
embodiment.
FIG. 6 is a cross-sectional view of said cam taken at 6--6 in FIG.
5.
FIG. 7 is a diagrammatic view of the steps of said cam.
FIG. 8 is a plan view of the internal support plate of said
embodiment.
FIG. 8A is a cross-sectional view taken at 8A--8A in FIG. 8.
FIG. 9 is a plan view of the permanent-magnet disk of said
embodiment, showing the ten pairs of poles on said disk.
FIG. 10 is a cross-sectional view of said disk taken at 10--10 in
FIG. 9.
FIG. 11 is a diagrammatic view of said embodiment in which the
positions of the external adjusting electromagnets are shown (much
smaller than actual size).
FIG. 12 is a diagrammatic view of said embodiment implanted beneath
the scalp and covered by an external adjustment element.
FIG. 13 is a diagrammatic view similar to FIG. 11 except that the
rotor and cam have been removed to show the magnetic polarization
of the four stator elements.
FIGS. 14 and 15 show the magnetic polarization of an alternative
embodiment, where a one-piece stator element is used.
FIG. 16 is a partial plan view, somewhat diagrammatic, of an
alternative embodiment wherein the stator elements each include an
electrical coil.
Structure
There is shown in FIG. 1 a shunt valve assembly 10 with two shunt
valves 12, 14 separated by a pumping chamber 16. Cerebroventricular
catheter 18 is connected to the inlet of the valve assembly, and
drainage catheter 20, to the outlet. This assembly can be
surgically implanted following well-known procedures.
A cross section through the downstream shunt valve 14 is shown in
FIG. 2. The upstream valve 12 is preferably the same except that
the adjustment mechanism is absent. (The tubular plastic covering
shown tightly fitted around the valves in FIG. 1 is not shown in
the remaining figures.) Valve body 22 (injection molded from a
surgically-implantable material such as polyethersulfone) has
within its interior an inclined plate 24 made from a nonmagnetic
material, such as titanium or stainless steel. The plate 24 has
circular aperture 26 in which is press fit a sapphire ring 28, with
frustoconical surface 30 forming a valve seat for spherical ball 32
(highly-polished ruby).
Biasing the ball against the valve seat is spring 34 (single piece
of stainless steel or another suitable material), shown in plan
view in FIG. 3. The spring provides a low K factor to produce
little change in working pressure with changes in flow (i.e., a
flat flow-pressure curve). The spring has base 36 overlying ball
32, central arm 38 extending from the base to an adjustment
mechanism, and two flanking arms 40, 42 extending from the base to
a yoke 44. The yoke is press fit into a hole in plate 24 and tabs
46 extend over the tops of the flanking arms. The yoke is relieved
in the center to provide room for the central arm to pass through.
Notches (not shown) cut in the ends of flanking arms 40, 42 receive
portions of the yoke, and secure the spring against longitudinal
movement. The spring is secured against sideward movement by
contact of the flanking arms with the vertical outside surfaces of
the yoke.
Plate 24 is held tightly in place within valve body 22. The tight
fit is achieved by sliding the plate into the valve body (in a
direction from right to left in FIG. 2). Grooves 54, 56 at the
upstream end of the valve body receive portions 50, 52 (FIG. 8) of
the plate, and grooves 49 at the downstream end receive tabs 48 on
the plate. The grooves extend generally horizontally rather than in
the inclined direction followed by the plate, and thus the tabs 48
and portions 50, 52 tend to become tightly wedged into the
grooves.
Grooves 54, 56 at the ball end of the valve body also serve to
press plate 24 downwardly so as to squeeze it tightly against
O-ring 58 (silicone rubber), which provides an internal seal to
ensure that all flow through the valve is through the orifice
formed between the ball 32 and the valve seat 30. Flow through the
valve is from inlet cavity 60, past ball 32, and into outlet cavity
62.
The preload of spring 34 against ball 32 is adjusted by using cam
66 (Delrin) to vary the vertical position (through a 0.75 mm range)
of free end 64 of central arm 38. The spring preload establishes
the pressure of the valve. The cam (best shown in FIGS. 5-7) has a
circular staircase of eighteen steps, each being grooved so as to
have a V-shape cross section. Free end 64 of arm 38 has a similar
V-shape chosen to mate with the V-shape of steps 68. At each end of
the staircase a barrier is provided by element 70. This confines
rotation of the cam to slightly less than one revolution. The
V-shape of steps 68 act as detents to keep the cam in precisely one
of eighteen possible angular positions. That means that the
vertical position of free end 64 of arm 38 is always at precisely
one of eighteen different values and, in turn, that the working
pressure of the valve is always at one of eighteen possible
levels.
Cam 66 is press fit into the central hole in rotor 72 (4 mm
diameter), with a protrusion on the cam fitting into recess 73 in
the rotor to assure accurate angular positioning. The cam-rotor
unit rotates loosely on shaft 76, the base of which is press fit
into plate 24. The unit is retained by retaining element 77 secured
to the top of the shaft. The rotor is preferably made of platinum
cobalt or samarium cobalt (which may be plated with platinum to
improve corrosion resistance). The rotor has ten permanently
magnetic poles 74 of alternate polarity (FIGS. 9-10). At any one
angular position, the pole exposed on the top surface of the disk
is opposite that of the one exposed on the bottom surface.
Below rotor 72 there are fixed in place four stator elements 78
each made of a material that is magnetically soft and permeable,
and that is resistant to corrosion in the presence of cerebrospinal
fluid, which contains chlorides. Preferred materials include
magnetic stainless steel alloys and alloys of nickel, iron, and
molybdenum or cobalt. As shown in FIG. 8, the stator elements are
embedded in a plastic member 80, which is fixed to plate 24 by
means of shaft 76. The stator elements are shaped so that the
portion of each lying beneath the rotor matches the size of
permanent magnets 74. The portions of the stator elements lying
radially beyond the rotor are sized to match the area beneath the
rotor so that the boundary between poles, when the stator is
magnetized, is at the perimeter of the rotor.
Operation
In operation the shunt valve assembly shown in FIG. 1 is surgically
implanted in a patient following well-known procedures. Before
implantation the pressure of adjustable valve 14 can be set to the
desired level according to the circumstances of the case. For
instance, it can be set approximately equal to the patient's
pre-operative ventricular CSF pressure so that no immediate
pressure change occurs as a result of the operation. After the
patient has recovered from the trauma of the operation, the
pressure is adjusted downwardly to the desired level. In the case
of normal-pressure hydrocephalus, the pressure is lowered to a
level sufficient to initiate shrinkage of the cerebral ventricle.
Further adjustments in pressure can be made at subsequent times, as
necessary. In the typical treatment of normal-pressure
hydrocephalus, the pressure would be adjusted upwardly after
sufficient shrinkage of the ventricle has occurred in order to
stabilize ventricle size.
In children at the beginning of the treatment, the pressure should
be lowered to a level inversely proportional to the ventricle size
to reduce stress on the brain parenchyma (see FIG. 13 of Hakim et
al., "The Physics of the Cranial Cavity", Surg. Neurol., Vol. 5,
March 1976), and as the ventricle decreases in size the pressure of
the valve should be increased, so when the ventricle attains normal
size the intraventricular pressure is again normal, thereby
avoiding development in the patient of a slit-ventricle condition.
Also in cases of normal-pressure hydrocephalus, sometimes in spite
of a low-pressure valve the patient does not improve and the
ventricle size remains unchanged, making the surgeon think he is
dealing with a case of brain atrophy. But by further changing the
valve pressure to a lower one, the ventricle decreases in size and
the patient immediately starts to improve. In elderly persons and
in long standing cases of normal-pressure hydrocephalus, it has
been found that the intraventricular pressure needs to be lowered
more than in young people and in hydrocephalus of short
duration.
Another advantage of the invention arises in the procedure for
determining when an implanted shunt valve can safely be removed
from a patient, i.e., determining whether the patient is still
dependent on the valve for drainage of excess cerebrospinal fluid.
The conventional technique for making that determination has been
to temporarily pinch closed the tube downstream of the valve and
observe the patient for symptoms (e.g., slight headache) indicative
of valve dependency. In the absence of symptoms the valve can be
removed. With the invention it is unnecessary to stop flow
entirely. A safer procedure can be followed. Valve pressure is
raised, slightly at first, more so later for confirmation, using
the adjustment mechanism.
Valve pressure adjustments are made by applying a pulsed magnetic
field to the vicinity of the shunt valve as shown diagrammatically
in FIGS. 11-13. A valve adjustment element 90 is applied over
adjustable valve 14 in the orientation shown. The adjustment
element contains four electromagnets 92, 93, 94, 95, which are
separately controlled by an external control device, shown
diagrammatically at 96. Adjustment element 90 has a marking (such
as an arrow pointing in the direction of CSF flow) on its exterior
to assure that it is applied to the valve in the correct
orientation, and it has a groove 98 in its bottom surface sized to
fit over the protrusion in the scalp, at the site of the implanted
valve. The groove is narrowed at one end 99 to enable correct
longitudinal alignment relative to the adjustable valve 14.
Control device 96 has input keys, which the operator uses to select
one of 18 possible desired pressures (from 20 to 190 mm H.sub.2 O)
and a pressure display.
Each of electromagnets 92, 93, 94, 95 can be energized to have
either the north or south polarity facing the stator elements, or
each can remain off altogether. Movement of rotor 72, in the
desired direction and through the desired angle, is achieved by
energizing the electromagnets in the sequence shown in the table in
FIG. 11. For example, clockwise motion is achieved by first
energizing electromagnets 92, 93 to south and north polarities,
respectively, and leaving electromagnets 94, 95 off. In the next
step electromagnets 92, 93 are left off, and electromagnets 94, 95
are energized to north and south polarities, respectively. The
sequence repeats itself after the fourth step. Rotor 72 is shown in
FIG. 11 in the position reached after the first step (the
polarities of the rotor magnets are those on the bottom surface).
If the magnetic field provided by the electromagnets is described
by a vector pointing from the south to the north pole of the
energized magnets, then it can be seen that the sequence prescribed
for causing rotor 72 to rotate clockwise (down the table in FIG.
11) amounts to rotating the field vector in the counterclockwise
direction (opposite that of the rotor), in 90.degree. steps.
Electromagnets 92, 93, 94, 95 are positioned 90.degree. apart and
spaced equal radial distances from a central axis. When adjustment
device 90 is installed properly over valve 14, the central axis of
the electromagnets is coincident with the axis of rotation of rotor
72, and each electromagnet is aligned at the same angular position
as one stator element 78. It is not, however, necessary that this
alignment be exact. The invention is tolerant of alignment errors,
which are unavoidable owing to the inability of the user to see
rotor 72 or stator elements 78 and to the small size of those
elements relative to the size of the external electromagnets.
The magnetic polarization induced in the stator elements 78 as the
result of energizing the electromagnets is diagrammatically
illustrated in FIG. 13. The two stator elements along the axis
connecting the two energized electromagnets are polarized in the
radial direction, so that the boundary between the poles lies
roughly at the peripheral edge of disk rotor 72. The radially inner
portions of these two stator elements, the portions lying beneath
rotor 72, have the opposite polarity of the portions lying outside.
By contrast, the stator elements along the other axis are polarized
so that the boundary between poles lies along the radial direction.
Both poles extend beneath the rotor 72. This pattern of
polarization will result even if there is substantial error in the
orientation of the electromagnets.
Movement of rotor 72 is influenced predominantly by the stator
regions 100 (shown in dashed lines in FIG. 13) lying beneath the
rotor, as it is those portions that are closest to the permanent
magnets 74 of the rotor. Accordingly, the part of the stator
elements with uniform polarity dominate over those with split
polarity. This phenomenon could be emphasized by making the stator
elements of a magnetically anisotropic material so that the
magnetization induced by the external electromagnets is strongest
along the radial axis of the corresponding stator elements.
The number of magnetic poles 74 is selected so that when one pair
of radially opposite stator elements 78 is aligned with one pair of
magnetic poles 74 (as are the upper left and lower right stator
elements in FIG. 11) the other two stator elements (the upper right
and lower left in FIG. 11) are each staggered halfway between two
of the poles 74. In operation, control device 96 energizes the
electromagnets closest to the pair of stators staggered between two
magnets, thereby causing the rotor to move through an angle
corresponding to one half the width of a magnetic pole 74.
In the preferred embodiment there are ten magnetic poles on each
side of the disk, and thus twenty angular increments in one full
revolution (i.e., each step is one twentieth of 360.degree., or
18.degree.). Only eighteen of these increments are used,
corresponding to the eighteen detented steps along the staircase
surface of cam 68 (the other two increments are occupied by the
detent wall 70 of the cam).
After a pressure is prescribed on control device 96, an enter key
is pressed. That initiates a sequence of eighteen steps in the
direction of lower pressure settings, counterclockwise rotation of
rotor 72. This assures that the cam is returned to a position
wherein spring arm 64 is at the lowest step on the cam staircase.
If fewer than eighteen steps are actually needed to bring the cam
to this position (as will most often be the case), the detent wall
provided by element 70 of the cam prevents further rotation. After
the eighteen-step resetting sequence is complete, the rotor is
moved clockwise by the number of steps corresponding to the
prescribed pressure.
Other Embodiments
Other embodiments are within the scope of the following claims, for
example:
The invention could be used in nonmedical applications.
A magnetically anisotropic material could be used for the stator
elements, with the strongest axis of magnetization oriented along
the radial direction. Such anisotropy could also be achieved
mechanically by splitting each stator element along the radial
direction into two or more segments.
A variable reluctance or hybrid rotor could replace
permanent-magnet rotor 72.
Linear movements within an implanted device could be achieved
following the invention by providing a linearly-moving element as
the rotor and by placing stator elements along the path of the
linearly-moving rotor.
A rotor with fewer poles could replace the ten-pole rotor of the
preferred embodiment, particularly where fine angular precision is
not required (e.g., in a pump a simple two-pole rotor might
suffice). Strong permanent magnets could be used to apply the
external field (e.g., in the two-pole rotor of the pump application
just described).
Electrical wire could be wrapped around the implanted stator
elements forming a coil so that an electrical current is induced
therein by the externally-pulsed magnetic field; the electrical
current would in turn magnetize the stator elements if the coil
circuit is closed by a resistor or a capacitor.
A single-piece stator element, e.g., with four lobes as shown in
FIG. 14, could replace the stator elements of the preferred
embodiment. An advantage of using a single-piece stator element is
that the stator can lie entirely inside of the outer perimeter of
the rotor, and thus provide for a more compact implanted unit. This
is because, when magnetized, as shown in FIG. 14, the dominating
lobes are all of one polarity, rather than split radially into two
polarity regions as in the preferred embodiment. Thus the entire
lobe, not just the inner half, can be influential in moving the
rotor. The disadvantages of the single-piece stator is its lower
tolerance to errors in alignment of the external magnetic field.
The reduced tolerance for misalignment can be understood by
reference to FIG. 15, in which the external field has been rotated
sufficiently to cause the lobes that were split into two poles to
now be entirely within the region of one pole. As a result all four
lobes have nearly equal influence on the rotor, and it is not
possible to move the rotor.
A presently even more preferred embodiment is one having a rotor 72
with six poles rather than ten (as shown in the figures). Such a
rotor provides twelve steps of 30.degree. for one full revolution.
Eleven of these can be used to provide eleven different pressure
settings (each differing by 15 mm H.sub.2 O) from 30 to 180 mm
H.sub.2 O. An advantage of six poles is that with the
four-stator-element configuration (FIG. 11) greater torque is
available using six rather than ten poles. This results because all
four stator elements, when magnetized by the external field,
generate a torque in the same direction. In the ten-pole embodiment
(FIG. 11), this is not the case. The torque generated by the stator
elements with split polarity (upper right and lower left in FIG.
11) is opposite, though weaker, that generated by the stator
elements with uniform polarity. It is instructive to note, however,
that this difference between the six and ten-pole embodiments is
just the opposite if a one-piece, four-lobed stator is used. In
that case, the split polarity stator elements generate opposing
torque with the six-pole rotor and not with the ten-pole one. Also,
if a ten-pole rotor is used with separate stator elements, torque
can be increased by using anisotropic material for the stator
elements, as that will increase the dominance of the uniform
polarity stator elements over the split polarity ones.
* * * * *